THE UNDERLYING IDEA 83 



while these hatched out, and the young were on the 

 nest until they were old enough to fly ; but before they 

 had left she had slipped a fresh egg among them, 

 ready to start a new batch. Whenever I saw the nest 

 throughout the entire summer, I found in it either 

 eggs, or young, or both." Such reproductive energy 

 as this is hard to beat; compared with this rate of in- 

 crease, the ordinary bird is the exponent of race sui- 

 cide. How can a robin hope to compete with this 

 family industry? What can a bluebird offer that will 

 approach such chances of a worthy successor when his 

 work shall be finished ? 



These, then, are the most important points in which 

 the English sparrow has varied from his sparrow 

 cousins and made of himself the most successful town 

 dweller in the bird world. He has become clannish 

 and gained the advantages of cooperation. He has 

 used man's highways and cars by means of which to 

 expand his area. He has cultivated the presence of 

 man and thus gained protection from his enemies, 

 food from man's waste, and nesting sites on man's 

 house. He has assumed a varied diet. The male has 

 become handsome. He has given up migrating, and 

 thus secured the best nesting sites. He has learned 

 to produce many offspring. With all his versatility, 

 why should he not succeed? 



Thrown into competition with our native birds, he 



